Photovoltaic cells are a means for converting incident radiation, typically actinic radiation such as solar radiation, into electrical energy. There is significant interest worldwide in the generation of power from sunlight. Radiation concentration is finding increasing relevance for the purpose of improving the efficiency or applicability of photovoltaic (PV) cells used for power generation for three reasons: first, because for some PV technologies, higher incident illumination intensity actually increases the efficiency of power conversion; second, because it is believed that for some applications, it may prove to be a more cost-effective approach for large area power generation and third, because for some applications, it is more practical to deploy concentrators than photovoltaic cells.
Many PV cells, however, require for their manufacture materials that are expensive and energy-intensive to produce. Accordingly, in order to improve the operating efficiency, applicability and cost-effectiveness of solar PV cells, light concentration approaches are being actively developed. Solar concentrators collect light over a large area and concentrate the light down onto a small area of photovoltaic cell (pv).
Two broad classes of solar concentrator are being developed. A first is a geometric solar concentrator, which can take the form of a reflective or refractive concentrating element. A reflective or refractive (geometric) solar concentrator operates by efficiently redirecting or focusing solar radiation incident on concave reflective surfaces or lenses onto a solar PV cell or cell array. A refractive solar concentrator operates by optically focusing radiation incident on a large lens surface onto the smaller PV cell or cell array surface. Whilst this approach has met with some success, the light concentrator optical elements, typically mirrors or lenses, need to be very robust and may not themselves be cheap items. Furthermore, the greater the concentration ratio, the smaller is the acceptance angle to the concentrator with the result that high-concentration systems also have the disadvantage that they are required to track the sun for efficient concentration and also are not very effective in diffuse light (e.g. cloudy weather).
The second class might be termed absorptive-emissive concentrators and act by absorbing the incident radiation and re-emitting radiation to a PV cell or cell array. These are also widely referred to as luminescent concentrators.
The absorptive-emissive or luminescent form of concentrator typically comprises a sheet of radiation-receptive material, the sheet itself being typically transparent, doped with a material capable of absorbing the incident radiation and then re-emitting radiation, which is typically a luminescent material, e.g. fluorescent material. The emitted radiation may then be directed via a waveguide to a PV array, typically positioned at the edge of the sheet (and thereby covering a much smaller area than if employed as the direct radiation absorber). The waveguide, which directs the re-emitted radiation to the edge of the sheet, is typically the sheet itself, by trapping the re-emitted radiation within the sheet by internal reflection. The absorptive-emissive radiation concentrators have the advantage that they do not need to track incident radiation for effective trapping of incident radiation and they are also effective in diffuse light.
The absorbing materials are typically fluorescent dyes or pigments which absorb energy within the solar spectrum and efficiently re-emit in a relatively narrow bandwidth, typically at a longer wavelength. A significant proportion of the re-emitted radiation (typically at least 70%) is trapped within the waveguide formed by the sheet by total internal reflection and may thereupon impinge upon the PV element configured at the edge of the sheet, which can then convert the radiation into electrical energy. The remainder of the emitted radiation (typically up to 30%) escapes from the waveguide and is lost.
There are several advantages associated with these absorptive-emissive or luminescent concentrators over geometrical concentrators. Most notably, there is a lesser requirement to track incident radiation in order to receive high concentration of light. Further, the collection efficiency is high even in diffuse light conditions. Large areas of luminescent solar concentrator enable good heat dissipation to air. Furthermore, the efficiency of a photovoltaic device or other radiation receiving device can be optimised by selecting a luminescent material (e.g. dye) which re-emits at a wavelength closer to the maximum spectral sensitivity of the PV or other receiving device. Luminescent planar concentrators are typically made of relatively low-cost materials (as compared with reflective concentrators for example) and are less visually intrusive.
There are, however, several problems with this form of absorptive-emissive or luminescent radiation concentrator, associated with the difficulty in finding suitable fluorescent dyes as absorbing materials. Several requirements have been identified for effective and efficient radiation concentration using absorptive-emissive systems. The absorbing material must be capable of: efficiently absorbing across the range of wavelengths of the incident radiation; emitting radiation at a wavelength suitable for absorption by the energy converter (e.g. photovoltaic element); emitting radiation with a high quantum yield (by which it is meant the ratio of the number of emitted to absorbed photons is close to unity); and not re-absorbing emitted radiation as it propagates through the waveguide. It is also required that the radiation absorber remains stable under solar illumination.
Typical organic fluorescent dyes having broad band absorption and emission have absorption and emission spectra which have significant levels of overlap, which results in re-absorption of emitted radiation. Re-absorption of emitted radiation reduces the efficiency for at least two reasons: when the reabsorbed radiation is again re-emitted, the quantum yield of the absorption-emission process results in a small loss if it is not unity; and up to approximately a further 30% is lost by being re-emitted in a direction which avoids total internal reflection by the waveguide. The latter loss arises because when re-emission occurs, the direction of propagation of the original light is lost and light is emitted in random directions resulting in the said further 30% loss of radiation from the system. If, on average, several of these interactions occur as light propagates from the centre of a luminescent planar concentrator to the edge, the intensity will be significantly reduced. This mechanism is the greatest source of loss in an LPC and restricts the size (and therefore benefit) of planar concentrators. This may have the effect of reducing the area of effective solar collection to areas close to the edge of the radiation receiver (luminescent planar concentrator) near the PV element.
Many attempts have been made to address this problem by selection of luminescent, typically fluorescent, dyes having a large Stokes shift (i.e. the wavelength difference between peak absorption and peak emission in the spectra). Whilst the Stokes shift in principle reduces the risk of reabsorption and increases the mean free path of re-emitted radiation, there typically remains a degree of overlap which limits the size of the concentrator that is possible before reabsorption/re-emission losses become significant.
There have been several attempts to overcome the difficulties associated with such fluorescent absorptive-emissive systems.
For example, in U.S. Pat. No. 4,110,223, there is described a multiple layer collection device, each layer acting as an independent solar concentrator and doped with a separate fluorescent dye having a relatively narrow bandwidth of absorption and a narrow emission bandwidth. By this method the effective absorptive bandwidth of the multiple layers covers a broad range of wavelengths. However, the disadvantages with this method are that the edge-mounted PV element is required to be three times the size (to cover three edges) and it is difficult to identify appropriate fluorescent dyes that absorb at different wavelengths but emit at the same narrow wavelength suitable for the PV element whilst meeting the other requirements of transparency, photo-stability, high Stokes shift, etc.
U.S. Pat. No. 4,188,239 describes a solar concentrator comprising a planar waveguide at least one edge of which impinges upon a photovoltaic cell, the waveguide comprising an active luminescent species responsive to a portion of the incident solar radiation to generate luminescent radiation trapped within the waveguide and delivered to the photovoltaic cell by total internal reflection. The device further comprises a backing layer comprising a mirror having deposited thereon a rough, diffusing layer of particulate solid inorganic phosphorescent material, activated by the shorter wavelength solar radiation not absorbed by the luminescent species in the waveguide. The phosphorescent material produces on activation a longer wavelength emission that is reflected back into the waveguide and is of a wavelength that may activate the luminescent material therein. The specific example described uses the reflective phosphorescent particulate layer to reintroduce transmitted incident radiation into the waveguide at a longer wavelength, whilst the waveguide contains two fluorescent materials for generating the fluorescence to be captured by the photovoltaic cell. Whilst this solution assists in re-capturing incident radiation outside the spectrum of activation of the luminescent material contained within the waveguide, the luminescent material itself, which in the specific example is sulforhodamine 101 organic fluorescent dye, remains unsatisfactory for use in the waveguide in that there is insufficient separation between the absorption and emission spectra, which leads to an unsatisfactory overlap and significant re-absorption.
A further problem with fluorescent dye-based systems has been the tendency for the dye to degrade over time due to exposure to solar ultraviolet light, although some efforts to identify more stable fluorescent dyes have been made.
U.S. Pat. No. 6,476,312 (Barnham et al) attempts to overcome the shortcomings of absorptive-emissive radiation concentrators that use organic fluorescent dyes as the absorbing materials and describes a radiation concentrator for use with a photovoltaic device, which comprises a wave-guide containing a plurality of quantum dots. The quantum dots cause a red-shift of incident radiation which is internally reflected by the waveguide to a waveguide output. Quantum dots are said to be of particular benefit due to their luminescent efficiency and the tenability of absorption thresholds and size of red shifts. The use of quantum well cells can tune the band-gap. According to U.S. Pat. No. 6,476,312, by incorporating quantum dots of a certain spread of sizes, the red-shifted radiation can be controlled to minimise overlap with the absorption spectrum and match the required bandwidth of the photovoltaic element. Whilst quantum dots possess the characteristic of suitable broad-band visible absorption and narrow band emission, they suffer from the common characteristic of small Stokes shift, which reduces the path length of emitted radiation due to re-absorption. Whilst efforts to increase that path length via controlling the spread of size of quantum dots have been described, the practical efficiency has yet to be demonstrated (e.g. Gallagher et al, Solar Energy 81 (2007) 813-821), the assumption being that whilst a spread of dot sizes increases the red shift of the absorption and emission peaks, the absorption spectrum becomes broader providing some overlap with the emission spectrum.
It would be desirable to provide a luminescent or absorption-emission concentrator system having improved efficiency and which addresses the problem or reabsorption of radiation within the device and which thereby improve PV absorption efficiency whilst overcoming the problems with prior art systems.